Organic light emitting devices (OLEDs) are light emitting devices that are comprised of several layers, in which one of the layers is comprised of an organic material that can be made to electroluminesce by applying a voltage across the device. Certain OLEDs have been shown to have sufficient brightness, range of color and operating lifetimes for use as a practical alternative technology to LCD-based full color flat-panel displays (S. R. Forrest, P. E. Burrows and M. E. Thompson, Laser Focus World, February 1995). Furthermore, since many of the organic thin films used in such devices are transparent in the visible spectral region, they allow for the realization of a completely new type of display pixel in which the red (R), green (G), and blue (B) emission layers are placed in a vertically stacked geometry to provide a simple fabrication process, a small R-G-B pixel size, and a large fill factor.
A transparent OLED (TOLED) which represents a significant step toward realizing high resolution, independently addressable stacked R-G-B pixels has been reported in International Patent Application No. PCT/US95/15790. This TOLED had greater than 71% transparency when turned off and emitted light from both top and bottom device surfaces with high efficiency (approaching 1% quantum efficiency) when the device was turned on. The TOLED used transparent indium tin oxide (ITO) as the hole-injecting electrode and a Mg-Ag-ITO layer for electron-injection. A device was disclosed in which the Mg-Ag-ITO electrode was used as a hole-injecting contact for a second, different color-emitting OLED stacked on top of the TOLED. Each device in the stacked OLED (SOLED) was independently addressable and emitted its own characteristic color through the transparent organic layers, the transparent contacts and the glass substrate, allowing the device to emit any combination of color that could be produced by varying the relative output of the red and blue color-emitting layers.
Thus, publication of PCT/US95/15790 provided the disclosure of an integrated OLED where both intensity and color could be independently varied and controlled with external power supplies in a color tunable display device. As such, PCT/US95/15790 illustrates a principle for achieving integrated, full color pixels that provide high image resolution, which is made possible by the compact pixel size. Furthermore, relatively low cost fabrication techniques, as compared with prior art methods, may be utilized for making such devices.
Devices whose structure is based upon the use of layers of organic optoelectronic materials generally rely on a common mechanism leading to optical emission. Typically, this mechanism is based upon the radiative recombination of a trapped charge. Specifically, devices constructed along the lines discussed above comprise at least two thin organic layers separating the anode and cathode of the device. The material of one of these layers is specifically chosen based on the material's ability to transport holes (the "hole transporting layer"); the other, according to its ability to transport electrons (the "electron transporting layer"). The electron transporting layer typically comprises the electroluminescent layer. With such a construction, the device can be viewed as a diode with a forward bias when the potential applied to the anode is higher than the potential applied to the cathode. Under these bias conditions, the anode injects holes (positive charge carriers) into the hole transporting layer, while the cathode injects electrons into the electron transporting layer. The portion of the luminescent medium adjacent to the anode thus forms a hole injecting and transporting zone while the portion of the luminescent medium adjacent to the cathode forms an electron injecting and transporting zone. The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, a Frenkel exciton is formed. Recombination of this short-lived state may be visualized as an electron dropping from its conduction potential to a valence band, with relaxation occurring, under certain conditions, preferentially via a photoemissive mechanism. Under this view of the mechanism of operation of typical thin-layer organic devices, the electroluminescent layer comprises a luminescence zone receiving mobile charge carriers (electrons and holes) from each electrode.
It would be desirable if each of the color-emitting OLEDs that are used in a SOLED could have the electroluminescent emission efficiently produced in a relatively narrow band centered near selected spectral regions, which correspond to one of the three primary colors, red, green and blue. The materials that are responsible for producing the electroluminescent emission are frequently incorporated into the OLED such that they also serve as the electron transporting layer of the OLED. Such devices are referred to as having a single heterostructure. Alternatively, the electroluminescent material may be present in a separate emissive layer between the hole transporting layer and the electron transporting layer in what is referred to as a double heterostructure.
In addition to having the emissive material present as the primary material in the electron transporting layer, the emissive material may also be present as a dopant that is contained within a host material. Materials that are present as host and dopant are selected so as to have a high level of energy transfer between the host and dopant materials. In addition, these materials need to be capable of producing acceptable electrical properties for the OLED. Furthermore, such host and dopant materials are preferably capable of being incorporated into the OLED using starting materials that can be readily incorporated into the OLED using convenient fabrication techniques.